The invention relates generally to the fabrication of a diamond coating or free standing products and, more particularly, to the fabrication of such coatings on the surface of various substrates, such as of milling cutters, bites (inserts), end mills and drills each having an excellent scale-off (or peeling-off) resistance, various abrasion (wear) resistant members such as valves and bearings, tool steels, as well as substrates acting as heat radiating substrates, such as heat sinks for electronic parts.
As is generally well known, applying a diamond coating to a substrate may be desirable to enhance the performance or to expand the applications of the original (uncoated) substrate.
As used herein, a diamond coating is a coating of carbon primarily in the SP.sup.3 phase.
The unique properties of diamond and the possibility to apply a coating thereof on a substrate offer the potential to exploit these properties in a wide range of applications. The combination of the highest hardness and highest thermal conductivity makes diamond a most effective material for abrasive, cutting, shaping and finishing tools. Its high thermal conductivity, high electrical resistance and low thermal expansion make it a preferred choice for spreading and conducting heat out of high power electronic devices. The chemical stability in corrosive environments and high wear resistance make it possible to use diamond as a protective coating in adverse environments.
Synthetic diamonds are produced by high pressure, high temperature techniques since the 1950's. They are now used in cutting, grinding and polishing. However, the development of techniques to produce diamond films and coatings starting with chemical vapor deposition (CVD) in the early 1980's opened up a vast field of applications. The most relevant applications of diamond coating are in the field of working tools and wear resistant coatings. The high thermal conductivity of these films can decrease wear significantly by rapid heat transfer from hot spots caused by frictional heating. Yet, diamond films have small coefficients of friction particularly in low humidity environments. Examples of wear resistant applications of diamond films include machine tool guides, cutting blades, gears, seals, fuel injection nozzles for internal combustion engines, spindle bearings and shafts of milling machines and lathes, computer discs and turbine blades. Medical applications of diamond coatings include surgical knife edges, precision scalpels and surgical implants. Drilling and cutting tools are a vast potential area for diamond coatings on an cutting tool materials. Diamond coated inserts are used for machining non ferrous metals such as high silicon aluminum alloys, copper alloys green ceramics, composite materials including fiber glass and carbon/carbons. Diamond coated inserts can replace conventional inserts in most turning, milling and round tool applications.
Current processing technologies for thin and thick diamond films include chemical vapor deposition (CVD), such as plasma CVD (e.g., DC plasma, RF induction plasma and microwave plasma CVD); non plasma CVD (e.g., hot filament and laser enhanced CVD); and a hybrid process (vapor-liquid-solid (VLS) growth); and an interactive laser technique, also known as the QQC process. Some of these techniques are addressed hereinunder in greater detail.
The major problem areas in diamond coating in which improvement is still needed are lowering substrate temperatures; increasing growth rates; improved adhesion to a variety of substrate materials; control of thickness uniformity on irregular shapes. These problems are further addressed herein. The present invention is directed at providing a solution to all of these problems.
As already mentioned, one area of particular interest is the application of diamond coating to machine tools (e.g., to machine tool cutting inserts). Diamond is especially a tough (hard) material, wears well, and has thermal qualities which are beneficial in many applications. For many machining applications, the qualities of diamond are seemingly unsurpassed by any other available material.
Carbide has long been an established choice for use in cutting tools and inserts, especially for cutting (machining) ferrous, nonferrous or abrasive materials such as aluminum and its alloys, copper, brass, bronze, plastics, ceramics, titanium, fiber-reinforced composites and graphite. Various forms of carbide are known for tools and inserts, such as cobalt-consolidated tungsten carbide (WC/Co).
CVD Processes:
Fabricating diamond coatings utilizing chemical vapor deposition (CVD) processes are well-known. These CVD processes, however, suffer from various shortcomings, including (i) the requirement for a vacuum chamber in which to carry out the process; (ii) the requirement of performing the process in a gaseous environment (typically methane gas, or the like); (iii) "poisoning" of the coating when forming a diamond coating over a cobalt-containing substrate; (iv) the requirement to preheat the substrate; (v) the requirement of a pressurized environment; and (vi) relatively low rates of deposition.
Certain enhancements to the CVD process have been proposed, including the use of microwave plasma enhanced (MWPE) CVD process which takes place at relatively low temperatures and pressures, as compared with conventional PCD fabrication methods which utilize High Pressure and High Temperature ("HPHT") techniques. Using these processes, any insert shape can reportedly be uniformly coated, and the coated inserts can have sharp edges and chip-breaker geometries. Hence, these inserts are indexable and can provide from two-to-four cutting corners.
CVD-coated tools tend to have a relatively thin diamond layer (typically less than 0.03 mm), which tends to allow the toughness of the underlying substrate material to dominate in determining overall tool strength, even when shock-loaded. Hence, these CVD inserts tend to be able to handle a larger depth of cut (DOC).
A critical concern with any coated tool or insert is that the coating should exhibit good adhesion to the underlying base material (e.g., carbide).
Concerns with the prior art include (i) delamination (catastrophic failure); (ii) adhesive and abrasive wear resistance (diamond is often used as a milestone for evaluating wear resistance); (iii) toughness (carbide is often used as a milestone for evaluating toughness); (iv) flank wear; (v) Built Up Edge (BUE) heat; and (vi) edge integrity.
The coating should also be compatible with the material contemplated to be machined. For example, polycrystalline diamond coatings tend to have a very low corrosion resistance to the resins in certain composite plastics.
Another area of concern with respect to diamond coatings on tools is that a very hard diamond coating on a softer tool is very prone to failure from stress.
An area of paramount concern is poor adhesion, which would appear to be a result from the reliance of prior art diamond coatings on the mechanism of molecular bonding, as well as from instabilities inherent in formation of the diamond coating.
An example of a CVD coating process is growing diamond by reacting hydrogen and a hydrocarbon gas, such as methane, in a plasma and synthesizing a diamond structure either as a coating or a free-standing blank. Carbide tools may be coated with a thin film of diamond using closed-chamber arc plasma CVD.
There are a number of basic CVD deposition processes currently in use, for depositing diamond coatings. Generally, these processes involve dissociation and ionization of hydrogen and methane precursor gases, which are then passed over and deposited onto a heated substrate.
The need to heat the substrate in order to apply the coatings is, in many ways, counterproductive. Such application of heat can cause distortion of the substrate, and the loss of any temper (heat treatment) that had previously been present in the substrate.
For example, in the hot filament CVD method, a tungsten or tantalum filament is used to heat the precursor gases to about 2000.degree. C. Substrate temperature ranges from 600.degree.-1100.degree. C. Using hydrogen and methane precursors, deposition rates of 1-10 .mu.m per hour are possible.
In DC plasma CVD, a DC (direct current) arc is used to dissociate the precursor gases, and can provide higher gas volumes and velocities than other prior art processes.
Microwave CVD uses microwaves to excite the precursor gases, resulting in deposition rates of several microns per hour. Coatings deposited using this method are of very high purity, closer to pure diamond than the other techniques.
In the CVD process there is a need to significantly elevate the temperature of the substrate. Furthermore, there is a significant (2-5 hour) cooling time, during which time residual precursors (gas) deposit, like snowflakes, on the surface being coated. This results in a coating which has a very rough surface, as compared to the pre-coated surface, and which typically requires post-processing to achieve a smoother surface.
In addition, when depositing a diamond coating, e.g., from a vapor phase, an amorphous coating is typically formed, containing either SP.sup.2 -bonded carbon or SP.sup.2 -bonded carbon and SP.sup.3 -bonded carbon, with higher concentration of hydrogen.
CVD processes are generally limited in suitability to coating flat planar surfaces, or simple (non-complex geometry) round surfaces.
The size of the substrate that can be coated is limited by the size of the vacuum chamber in which the process is carried out; the size of the substrate is typically less than eight inches in diameter.
Inasmuch as these processes tend to rely primarily on a precipitation-type (i.e., generally directional deposition) mechanism, the "other" side of the substrate may exhibit shadowing or uneven deposition.
Irrespective of the process involved in applying a coating to a substrate, the end-product may still provide unacceptable results. For example, applying a thin hard coating over a soft substrate will result in very poor stress distribution.
Prior art coating processes tend to be limited to forming a thin film (or layer) on a substrate. This is somewhat analogous to rain falling on a lawn and freezing. The resulting ice layer is relatively hard, but is thin, and there is an abrupt transition of hardness from the thin ice layer (coating) to the underlying grass (substrate). This will result in extremely poor stress distribution, as a result of which the thin layer of ice is subject to cracking when stress is applied. Generally, the thickness of the coating will reflect upon the stresses that build up in the coating.
CVD coatings are typically grainy, although they can be post-process finished to provide a surface of desired smoothness. However, in order to perform such post-finishing, a diamond must be employed. Further, as in any abrasive process, there will be directional scratches, albeit microscopic, evident in the final surface finish of the coating.
Diamond is a material of choice for coating tools because of its extreme hardness (9000 kg/mm.sup.2) and its low coefficient of friction (0.05). However, regardless of the substrate material (e.g., cemented carbide) adhesion of diamond coatings has been a barrier to its widespread application. In the case of carbide substrates, these adhesion problems are augmented by the cobalt binder phase found in carbide tools which essentially "poisons" the diamond nucleation and growth process, resulting in formation of graphitic carbon (which is undesirable).
Attention is directed to the following U.S. patents, incorporated by reference herein, as indicative of the state of the art of diamond coating: U.S. Pat. Nos. 5,273,790; 5,273,825; 5,271,971; and 5,270,077 ('077). The '077 patent, for example, discloses contacting a heated substrate with an excited gaseous hydrogen and hydrocarbon mixture under conditions of pressure, temperature and gas concentration which promote the growth of a diamond coating on a convex growth surface of the substrate, then separating the diamond coating from the convex growth surface, to provide a flat diamond film. The diamond coating separated from the substrate is under stress, and may require further processing for certain applications. Due to internal residual stresses, the diamond layer may also be deformed.
Generally, the prior art techniques for applying a diamond coating to a substrate (e.g., tool insert), although useful, suffers from one or more of the following limitations (i) the CVD deposition rate is limited to approximately 0.5 .mu.m-10 .mu.m per hour; (ii) the diamond coating exhibits poor adhesion (e.g., 30 kg/mm .sup.2) on carbide substrates with higher cobalt content, requiring specialty substrates or other surface treatment; (iii) the processes are generally directed to the formation of amorphous DLC coatings only, containing SP.sup.2 and/or SP.sup.3 and non-diamond carbon phases (e.g., graphite); (iv) The CVD process typically requires the substrate to be heated to 450.degree.-1000.degree. C., to enable coating growth and bonding, which can distort the substrate and which can add significant on-line time (e.g., 2 hours) to the process; (v) the processes must be performed in a vacuum chamber, such as a belljar, which adds complexity to the process and which severely limits the size of the substrate to be coated; (vi) stainless steel and steel cannot be easily coated using these processes; (vii) the processes are generally not well-suited to coating large surfaces, or surfaces with complex geometries.
These and other limitations of the prior art are addressed by the techniques of the present invention, which do not depend upon a vacuum environment, and which do not require preheating the substrate to perform the basic coating.
Laser Processes:
Attention is directed to the following U.S. Patents, incorporated by reference herein, as representative of processes utilizing lasers to form, or to assist in forming, diamond or DLC coatings, crystals, or structures:
U.S. Pat. No. 5,273,788, which discloses a layer of a hydrocarbon molecule applied to a substrate by the Langmuir-Blodgett technique, and the surface is irradiated with a laser to decompose the layer of molecules at the surface without influencing the substrate; U.S. Pat. No. 5,236,545, which discloses a process involving the deposition of a cubic boron nitride (CBN) layer on a silicon substrate as a first interfacial layer using laser ablation with a hexagonal boron nitride target in a nitrogen-containing atmosphere., followed by a second interfacial layer of hydrogen terminated carbon which is deposited with laser ablation with a carbon target in the presence of atomic hydrogen, followed by deposition of a heteroepitaxial diamond film using convention al chemical vapor deposition (CVD) technique; U.S. Pat. No. 5,176,788, which discloses the use of pulsed laser light to join diamond structures together. The process includes forming a layer of opaque non-diamond material between the two diamond surfaces to be joined, pressing the diamond surfaces together, using the pulsed laser light to quickly melt all the opaque nondiamond carbon material before a significant amount of heat is lost through the diamond surface, then allowing the resulting carbon melt to cool and solidify as polycrystalline diamond which grows homoepitaxially from the diamond surfaces, bonding those surfaces together; U.S. Pat. No. 5,154,945, which discloses the use of infrared lasers to deposit diamond thin films onto a substrate. In one embodiment, the deposition of the film is from a gas mixture of CH.sub.4 and H.sub.2 that is introduced into a CVD chamber and caused to flow over the surface of the substrate to be coasted while the laser is directed onto the surface. In another embodiment, pure carbon in the form of soot is delivered onto the surface to be coated and the laser beam is directed onto the surface in an atmosphere that prevents the carbon from being burned to CO.sub.2 ; U.S. Pat. No. 5,080,752, which discloses a process in which particles of transparent diamond powders are bonded together by polycrystalline diamond to form useful diamond structures. An intimate mixture of fine opaque nondiamond carbon powder and transparent diamond powder is pressed together to form a green body that is confined in either a thin walled transparent quartz vessel or a polycrystalline diamond coating, and a pulse laser is used to quickly melt the opaque nondiamond carbon powder. Then, the carbon melt is allowed to cool and grow homoepitaxially from the surfaces of the diamond particles, producing a polycrystalline diamond that bonds the diamond particles together; U.S. Pat. No. 5,066,515, which discloses a method of forming an artificial diamond comprising applying a laser beam to a glassy solid carbon material while moving a point on the glassy solid carbon material at which the laser beam is applied, to form a locally fused portion thereon, whereby every part of the locally fused portion is cooled as the point moves away therefrom. During cooling of the locally fused portion, an artificial diamond is formed in adjacent regions on both sides of the solidified locally fused portion; U.S. Pat. No. 4,986,214, which discloses a thin-film forming apparatus capable of forming thin diamond films. The process is a laser CVD process in which thin-film forming gases are optically dissociated by high energy photons released form an ultraviolet laser beam; U.S. Pat. No. 4,981,717, which discloses generating a plasma by a laser pulse. The pulse is fired into a gas and is absorbed in an initiater mixed with the gas. The resulting detonation produces a plasma of ions, radicals, molecular fragments and electrons which is propelled by the detonation pressure wave to a substrate and deposited thereon; U.S. Pat. No. 4,954,365, which discloses preparing a thin diamond film by immersing a substrate in a liquid containing carbon and hydrogen, and then subjecting the substrate to at least one laser pulse; U.S. Pat. No. 4,948,629, which discloses depositing diamond films by CVD using a high powered pulsed laser and a vapor which is an aliphatic carboxylic acid or an aromatic carboxylic anhydride; U.S. Pat. No. 4,874,596, which discloses directing an intense radiation beam into a cavity supporting a small quantity of material to be reacted on. Two or more intense radiation beams, such as generated by one or more lasers or electron guns, are directed against a particle or pellet of material from opposite directions, causing shock waves which collapse against the pellet material, transforming it into another form. The pellet or particles may comprise carbon, which is converted to diamond by the intense heat and force of the shock wave(s); and U.S. Pat. No. 4,849,199, which discloses suppressing the growth of graphite and other non-diamond carbon species during the low pressure deposition of carbon to form diamond. The graphite or other non-diamond species is vaporized using incident radiative energy sufficient to vaporize graphite but insufficient to damage the substrate. The growth of graphite and other non-diamond species is suppressed during deposition of diamond by exposing growing surfaces to incident radiative energy of a wavelength sufficient to selectively photolyze non-diamond carbon-carbon bonds formed at the surface of the growing diamond; U.S. Pat. No. 4,522,680, which discloses a method of producing diamond crystals by providing a pressure-resistant body having a nucleus of a starting material being crystallized in the inside thereof. The nucleus is applied with an energy which is capable of passing through the pressure-resistant body and being absorbed by the starting material, by which the nucleus is heated and melts. The melt is then gradually cooled under pressure to form crystals. A laser beam or high frequency induction heating technique is used for heating the nucleus.
These processes fall short of meeting the objectives of the present invention in various ways.
What is needed is a method and apparatus for commercially and economically treating and/or fabricating objects to obtain a desired composite material. Furthermore, it is desirable to improve the deposition rate, bond strength, adhesion, process time, area of growth, and material strength. Furthermore, it is desirable to produce an object having a desired composite material so as to permit effective engineering evaluation of material strength and production of parts and produce parts that exhibit surfaces having precise dimensions according to engineering data.
EPD Processes:
Electrophoretic deposition of diamonds has been applied so far only for predepositing a nucleating layer of diamonds ("seeding") for CVD of diamond films. Thus, U.S. Pat. No. 5,128,006 describes a method for attaining uniform and adherent CV deposition of diamond layers on a silicon substrate by predepositing a nucleating layer of diamond particles using an electrophoretic technique. The invention disclosed in U.S. Pat. No. 5,128,006 relates also to use of masking techniques so the predeposited layers can be patterned. The inventors described the electrophoretic seeding technique in a paper in J. Electrochem. Soc. Vol. 138, No 2, 1991 p. 635.
Doug-Gu and Singh described synthesis of oriented diamond films using controlled seeding of diamond particles by electrophoresis (Appl. Phys. Lett. 70, (12) March 1997 p. 1542) on silicon substrates. The film was then grown by hot filament CVD.
In both cases very low concentrations of diamond particles were used (0.1-0.2 g/l) to obtain less than or equal to one monolayer with partial coverage of substrate surface.
At Sandia National Laboratories (SNL), Panitz et al. (Journal of Vacuum Science and Technology A12(4), pp. 1480-1486, 1994) formed diamond powder precursors on nickel sheets and silicon wafers by two methods: electrophoretic deposition and screen printing. They then densified these precursors using a CVD hot filament assisted reactor. The EPD was carried out from isopropanol suspensions containing 2.5% by weight diamond powder, at 1000 volts across a spacing of 1.5 cm. The substrate acted as an anode, since the diamond particles acquired a negative charge. The suspension was sonicated for 1 hr with a 40 KHz ultrasonic cleaner in order to yield stable active dispersions with reproducible properties. The suspensions had to be periodically sonicated. In this method the diamond particles tend to deposit on high field areas first. Conformal coatings form on shaped substrates after a certain amount of processing time. In addition, dispersions with more than 2.5% wt diamond tend to be unstable. Panitz et al. found that coatings thicker than 200-250 micron cannot be electrophoretically deposited.
There is thus a widely recognized need for, and it would be highly advantageous to have, a method for diamond coating devoid of the limitations of the prior art, and which has advantages in the following directions: lowering substrate temperatures; increasing growth rates; improving adhesion to a variety of substrate materials; control of thickness uniformity on irregular shapes.